In a Bayer process for producing aluminum hydroxide from bauxite as a raw material, approximately 860 kg of a bauxite dissolution residual substance is generated per 1 t of reduced mass of alumina as a product. Since such a large amount of a bauxite dissolution residual substance is generated, it is necessary to treat a bauxite dissolution residual substance quickly in order to produce aluminum hydroxide effectively.
In a Bayer process, a soluble alumina component contained in bauxite ore is usually dissolved and extracted at a high temperature with an alkaline solution such as a caustic soda or a sodium aluminate solution, and the resultant extracted slurry is subjected to solid-liquid separation into a bauxite dissolution residual substance and a sodium aluminate solution.
Since the separated bauxite dissolution residual substance exhibits strong alkali, it is subjected to treatment such as pH adjustment so as to be suitable for a final disposal method (sea dumping, an inning). The treated bauxite dissolution residual substance is usually exhausted in a slurry state outside the system.
Bauxite dissolution residual substance separated from sodium aluminate solution exhibits a pH of approximately 12-13, even after it is washed. In general, a bauxite dissolution residual substance is subjected to neutralizing treatment by adding mineral acid such as, for example, a sulfuric acid, hydrochloric acid is further added to the bauxite dissolution residual substance slurry.
However, a bauxite dissolution residual substance slurry has the property that the pH value of the slurry increases with the passage of time after neutralization. This is due to the fact that the neutralization decomposition rate of a sodalite contained in a bauxite dissolution residual substance is low, or to the fact that elution of alkaline components drawn into pores of sodalite crystal, such as sodium aluminate, etc.
For example, even if a bauxite dissolution residual substance slurry is neutralized so that the pH value thereof will be approximately 7, isolation and elution of Na2O components from non-reacted sodalite contained in the bauxite dissolution residual substance gradually occur, thereby increasing the pH value of the bauxite dissolution residual substance slurry. Thus, in the neutralization process of a bauxite dissolution residual substance slurry with acid, neutralization is performed aiming at a pH which is lower than desired pH value.
However, a bauxite dissolution residual substance slurry has a property that the bauxite dissolution residual substance slurry will be viscous to gelate in neutralizing, when the pH becomes approximately 4.5. Therefore it is not possible to aim at too much low pH value.
In addition, the amount of Na20 contained as a sodalight in a bauxite dissolution residual substance slurry varies greatly, depending on the raw material bauxite and operating conditions. For example, the quantity of sodalight often varies greatly depending on the amount of reactive silica contained in the bauxite as a raw material. In other words, it is not always possible to control the pH value so that it is within a desired range by simply adjusting the pH value, due to the sodalite contained in the bauxite dissolution residual substance slurry.
In order to solve this problem, the following Patent document 1 discloses a method for neutralizing a bauxite dissolution residual substance characterized in that the neutralizing treatment is divided into at least three steps and that the first step of neutralization is performed such that objective value of pH value is aimed at a value ranging from 7 to 10. In the Patent Document 1, it is disclosed that in accordance with this method, solidification of a bauxite dissolution residual substance slurry can be avoided, considering of material of the neutralization equipment becomes unnecessary, and the total amount of sulfuric acid required for neutralization can be greatly reduced, compared to those of the method for neutralizing comprising one step and the method for neutralizing comprising two steps. However, this method requires a long time because neutralization is performed at plural steps.
As a solution for solving this problem, the following Patent Document 2 discloses a method for neutralizing the bauxite dissolution residual substance slurry which comprises: a single step of the neutralizing bauxite dissolution residual substance slurry using a thin acid of 3N or less so as not to lower the pH value thereof, thereby preventing increases of viscosity and gelation.
In accordance with this method, the operation time for adding acid is shortened. However, as for neutralization and decomposition of a sodalite contained in the bauxite dissolution residual substance, the decomposition rate of Na20 contained in sodalite using a thin acid becomes low. Thus, it also takes a long time for the pH to become stable. In addition, in this method, the quantity of the treatment solution increases because the use of thin acid, a neutralizing reaction apparatus becomes large, thereby increases the cost of equipment. In addition, it requires concentration of the treated liquid, if handling such as the transportation of slurry after processing is considered.    [Patent Document 1] Japanese Patent Publication No. 59-15719    [Patent Document 2] Japanese Patent No. 3,663,647